Concrete and other masonry or cementitious materials have compressive strength but substantially low tensile strength. Thus, when using concrete as a structural member, for example, in a building, bridge, pipe, pier, culvert, or the like, it is conventional to incorporate reinforcing members to impart the necessary tensile strength. Historically, the reinforcing members are steel or other metal reinforcing rods or bars, i.e., "rebar". Such reinforcing members may be placed under tension to form prestressed or positioned concrete structures.
Steel and other metals are, however, susceptible to oxidation. For example, ferrous metal rusts by the oxidation thereof to the corresponding oxides and hydroxides of iron by atmospheric oxygen in the presence of water. Concrete normally is poured at a pH of 12 to 14 (i.e., at high alkalinity) due to the formation of hydroxides of sodium, potassium, and calcium on the hydration of concrete. As long as the pH is maintained, the steel is passive leading to long-term stability and corrosion resistance.
Lowering the pH or exposure to a strong acid such as chlorine ions can cause the steel to be corroded. For example, chlorine ions permeating into the concrete can cause corrosion. Sources of chlorine ions include road salt, salt air in marine environments, and salt-contaminated aggregate (e.g., sand) used in making the concrete. When the reinforcement corrodes, expansion can occur, resulting in internal stresses in the concrete. This leads to cracking of the concrete which begins to disintegrate. For example, a crumbling bridge structure will be characterized by large sections of concrete crumbled away, exposing rusted steel rebar reinforcements. Moreover, the cracking and crumbling concrete causes exposure of additional steel to atmospheric oxygen, water, and sources of chlorine ions.
Such structural damage has become a major problem in a wide variety of geographical areas. For example, bridges and other concrete building infrastructures in northern United States cities are constantly in need of repair because of the salting of roadways after each winter snowstorm. Another example is the bridges leading to the Keys in Florida which are exposed to sea air. These bridges are continuously being rebuilt because of the short lifespan of the concrete. Yet another example includes buildings in Saudi Arabia and the Middle East wherein concrete is typically made using the acidic sand of the region. Thus, it is readily apparent that there is a critical need for a solution to the corrosion problem.
Various solutions to the corrosion problem of steel rebar have been offered. These solutions, however, have been largely unsuccessful for various reasons. Noncorrosive coatings on the concrete or steel rebar or both have been proposed. For example, U.S. Pat. No. 5,271,193 to Olsen et al. proposes a steel-reinforced concrete product such as a manhole cover having a coating of a corrosion resistant gel coat layer and an intermediate layer of fiberglass between the concrete and the gel coat layer. The gel coat layer is described as being a "hardenable polymeric fluid material." U.S. Pat. No. 4,725,491 to Goldfein proposes steel rebar members having chemical conversion iron oxide coatings thereon such as black iron oxide. U.S. Pat. No. 5,100,738 to Graf proposes steel rebar having a first layer of a synthetic material (e.g., epoxy resin) and a second layer of aluminum or aluminum alloy between the first layer and the steel. These exemplary coatings, in general, tend to be expensive and have had mixed results and acceptance.
There has also been interest in replacing the steel with various fiber-reinforced resins. For example, U.S. Pat. No. 5,077,133 to Kakihara et al. proposes a first filament bundle spirally wound around a fiber-reinforced core, a plurality of second filament bundles positioned axially along the core and a third filament bundle spirally wound around the core and the other bundles. U.S. Pat. No. 4,620,401 to L'Esperance et al. proposes a fiber reinforced thermosetting resin core and a plurality of continuous fibers helically wound around the core and impregnated with the thermosetting resin. The fiber-reinforced rods proposed therein have manufacturing limitations and are difficult to manufacture continuously and rapidly. Additionally, the winding of filaments onto a core tends to reduce the tensile strength of the core and can cause wicking problems.
Thus, there continues to be a need for a synthetic reinforcing rebar to replace steel and metal rebar without sacrificing the physical properties attributed to steel and metal rebar.